The National September 11 Memorial & Museum opens with a ceremony Thursday in New York City, in the footprint of where the Twin Towers once stood. "It was very, very profound, very emotional — much more than I thought,'' former New York City Mayor Rudy Giuliani told Matt Lauer on TODAY Thursday of seeing it for the first time.

"In some ways, it's harder now to relive it than it was a year later, two years later, three years later, maybe because you don't think about (the memories) as often and when they come back, they're more powerful."

By the numbers, the sheer scale of the museum is breathtaking.

110,000: Square feet of exhibition space in the museum, located in the heart of the World Trade Center site.

$700 million: Cost of construction of the museum.

23,000: Photos and images collected connected to Sept. 11 that line the walls of the museum.

SHANNON STAPLETON / Reuters

"There are (also) plenty of images that show the heroism, the bravery, what the firefighters did, the police officers, the individual citizens, (and) how they helped each other," Giuliani said on TODAY Thursday.

10,300: Artifacts are on display, 2,380 of which were provided by donors to the museum.

500: Hours of video images can be seen.

STAN HONDA / AFP - Getty Images

"It gives you a sense of the enormity of the attack,'' Giuliani said of the museum, which has a model of the World Trade Center buildings.